Chemical vapor deposition (CVD) is a process that can be used to grow desired objects epitaxially. Examples of current product lines of manufacturing equipment that can be used in CVD processes include the TurboDisc®, MaxBright®, and EPIK™ family of MOCVD systems, manufactured by Veeco Instruments Inc. of Plainview, N.Y.
A number of process parameters are controlled, such as temperature, pressure and gas flow rate, to achieve a desired crystal growth. Different layers are grown using varying materials and process parameters. For example, devices formed from compound semiconductors such as III-V semiconductors typically are formed by growing successive layers of the compound semiconductor using metal organic chemical vapor deposition (MOCVD). In this process, the wafers are exposed to a combination of gases, typically including a metal organic compound as a source of a group III metal, and also including a source of a group V element (for example, arsenic or phosphorus) which flow over the surface of the wafer while the wafer is maintained at an elevated temperature. Generally, the metal organic compound and group V source are combined with a carrier gas which does not participate appreciably in the reaction as, for example, nitrogen, or hydrogen. One example of an III-V semiconductor is indium phosphide (InP), which can be formed by reaction of indium and phosphine or aluminum gallium arsenide (AlGa1-xAsx), which can be formed by the reaction of aluminum, gallium, and arsine. The reaction of the compounds form a semiconductor layer on a substrate having a suitable substrate. These precursor and carrier gases can be introduced by an injector block configured to distribute the gas as evenly as possible across the growth surface.
The wafer is usually maintained at a temperature on the order of 500-1200° C. during deposition of precursor gases and related compounds. The precursor gases, however, are introduced to the chamber at a much lower temperature, typically 200° C. or lower. Thus, as the precursor gases approach the wafer, their temperature increases substantially. Depending on the precursor gases used in deposition of the particular wafer under construction, pyrolyzation of the precursor gases can occur at an intermediate temperature between that of the input gases and the wafer. This pyrolyzation facilitates the interaction of the precursor gases and growth of the crystal structure.
In a MOCVD process chamber, semiconductor wafers on which layers of thin film are to be grown are placed on rapidly-rotating carousels, referred to as wafer carriers, to provide a uniform exposure of their surfaces to the atmosphere within the reactor chamber for the deposition of the semiconductor materials. Rotation speed is on the order of 1,000 RPM. The wafer carriers are typically machined out of a highly thermally conductive material such as graphite, and are often coated with a protective layer of a material such as silicon carbide. Each wafer carrier has a set of circular indentations, or pockets, in its top surface in which individual wafers are placed.
In some systems, the wafer carrier can be supported on a spindle within the reaction chamber so that the top surface of the wafer carrier having the exposed surfaces of the wafers faces upwardly toward a gas distribution device. While the spindle is rotated, the gas is directed downwardly, along an increasing temperature gradient, onto the top surface of the wafer carrier and flows across the top surface toward the periphery of the wafer carrier. The used gas is evacuated from the reaction chamber through ports disposed below the wafer carrier. The wafer carrier is maintained at the desired temperature and pressure by heating elements, typically electrical resistive heating elements disposed below the bottom surface of the wafer carrier. These heating elements are maintained at a temperature above the desired temperature of the wafer surfaces, whereas the gas distribution device typically is maintained at a temperature well below the desired reaction temperature so as to prevent premature reaction of the gases. Therefore, heat is transferred from the heating elements to the bottom surface of the wafer carrier and flows upwardly through the wafer carrier to the individual wafers. In other embodiments, the wafer carrier can be supported and rotated by a rotation system that does not require a spindle. Such a rotation system is described in U.S. Patent Application Publication No. 2015/0075431, the contents of which are hereby incorporated by reference herein. In yet other embodiments, the wafer carrier holding at least one wafer is placed face down (inverted) in the reaction chamber and the gas distribution device is situated below the wafer carrier such that the process gases flow upwardly towards the at least one wafer.
The precursor gas flow is generally downward (that is, perpendicular) to the surface of a wafer carrier along an increasing temperature gradient until it reaches pyrolyzation temperatures, then impinges upon the wafer surface(s) that are being grown. This permits for the growth of the crystalline structure on the wafer. In most systems, there is additional pyrolyzed gas that flows around the wafer carrier. If this pyrolyzed gas is not removed from the reactor, buildup of undesirable materials on surfaces can occur. Such buildup can build up on the reactor, or occasionally and unpredictably flake off, falling onto the wafer being grown. These events can damage the reactor or epitaxial layers being grown on the wafers. Therefore, pyrolyzed gas is conventionally removed from the reactor after passing over the wafer. Nonetheless, buildup of the pyrolyzed gas has been known to occur in the reactor, in particular on the radially outer portions of the reactor housing.